Once upon a time, NASA launched an inflatable disco ball into orbit. For nearly four years, Explorer 24 was used to determine atmospheric density in all its polka-dotted balloon glory.

Explorer 24: The Death Star Disco Ball Satellite

The 8.6 kilogram mass of aluminum foil and plastic film had an identical configuration to Explorer 9 (launched in 1961), and Explorer 19 (launched in 1963). For launch, the sphere was packed into a small tube, 21.6 centimeters in diameter and 48.3 centimeters long. The tube was mounted on the nose of the launch vehicle. After the final rocket stage, a nitrogen gas bottle inflated the sphere, and a spring ejected it into its own orbit. The inflatable sphere stretched to 3.6 meters in diameter, uniformly speckled with 5.1 centimeter white dots to provide thermal control.

Like Explorer 9 and 19, Explorer 24 carried a 136-MHz tracking beacon to make ground tracking simpler. The beacon was mounted on the spacecraft skin, using the balloon as an antenna. But, unlike the spacecraft before it, Explorer 24's beacon actually worked and had sufficient power from batteries and solar cells to be tracked without relying solely on the SAO camera network.

The satellite looks utterly ridiculous — if it isn't a disco ball, surely this is the first Death Star prototype. But, it worked! Sequential observations of the satellite's position over time were successfully converted into the atmospheric density at perigee.

Explorer 24 carpooled into orbit with Explorer 25, both launched by a Scout rocket fired out of Vandenberg Airforce Base in California. Explorer 25, also known as Injun 4, is a magnetically aligned satellite measuring the influx of energetic particles, atmospheric heating, geomagnetic activity, and radiation belts. It took until February 1965 for the satellite to reach magnetic alignment, and sent radiation data until December 1966. Unlike Explorer 24, Explorer 25 is still in orbit, and is expected to stay in orbit for about 200 years.

Image credit: NASA. Read more about the Explorer program, or learn about the most recent advance in weather satellites.